Photovoltaic cells are commonly used to convert energy in the form of light into energy in the form of electricity. Typically, a photovoltaic cell includes a photoactive material disposed between two electrodes.
One type of photovoltaic cell is a dye-sensitized solar cell (DSSC). Referring to FIG. 1, a DSSC 100 is shown, which includes a charge carrier layer 140 (e.g., including an electrolyte, such as an iodide/iodine solution) and a photoactive layer 145 (e.g., including a semiconductor material, such as TiO2 particles) disposed between an electrode 101 and a counter electrode 111. Photoactive layer 145 also includes a photosensitizing agent, such as a dye. In general, the photosensitizing agent is capable of absorbing photons within a wavelength range of operation (e.g., within the solar spectrum). Electrode 101 includes a substrate 160 (e.g., a glass or polymer substrate) and an electrically conductive layer 150 (e.g., an ITO layer or tin oxide layer) disposed on an inner surface 162 of substrate 160. Counter electrode 111 includes a substrate 110, an electrically conductive layer 120 (e.g., ITO layer or tin oxide layer), and a catalyst layer (e.g., formed of platinum) 130, which catalyzes a redox reaction in charge carrier layer 140. Electrically conductive layer 120 is disposed on an inner surface 112 of substrate 110, while catalyst layer 130 is disposed on a surface 122 of electrically conductive layer 120. Electrode 101 and counter electrode 111 are connected by wires across an external electrical load 170.
During operation, in response to illumination by radiation in the solar spectrum, DSSC 100 undergoes cycles of excitation, oxidation, and reduction that produce a flow of electrons across load 170. Incident light excites photosensitizing agent molecules in photoactive layer 145. The photoexcited photosensitizing agent molecules then inject electrons into the conduction band of the semiconductor in layer 145, which leaves the photosensitizing agent molecules oxidized. The injected electrons flow through the semiconductor material, to electrically conductive layer 150, then to external load 170. After flowing through external load 170, the electrons flow to layer 120, then to layer 130, and subsequently to layer 140, where the electrons reduce the electrolyte material in charge carrier layer 140 at catalyst layer 130. The reduced electrolyte can then reduce the oxidized photosensitizing agent molecules back to their neutral state. The electrolyte in layer 140 can act as a redox mediator to control the flow of electrons from counter electrode 111 to working electrode 101. This cycle of excitation, oxidation, and reduction is repeated to provide continuous electrical energy to external load 170.